Always feeling tired with muscle weakness

[RRM]

Yes, thats probably too little sugars replenished. It takes a while to adjust.

[MarciavD]

I still eat according to an eating window of 6-8 hours a day. Last two weeks it seems like my energy is dropping again, especially at the end of the day I feels like my muscles are totally empty and I simply feel exhausted. My walk in the morning sometimes takes some effort too..
Do you have any tips how to pinpoint the culprit / cause of this? Eating in a more narrow window or just juice + salmon, what could I try best?

[RRM]

especially at the end of the day I feels like my muscles are totally empty and I simply feel exhausted.

It's probably true.
There is nothing wrong with that though, as essential to trigger autophagy.

My walk in the morning sometimes takes some effort too..

That means that you did not replenish all lost sugars, probably.
That is not good, because all sugars need to be replenished.

Do you have any tips how to pinpoint the culprit / cause of this? Eating in a more narrow window or just juice + salmon, what could I try best?

Making the eating window smaller would make it harder to replenish all lost sugars.
Instead, try to avoid fiber.
Yes, OJ is most effective, and of course, you need to eat sufficient salmon (about twice as much compared to when not fasting), as fasting also depletes blood protein (amino acids) levels.